The present invention relates to a method of producing bismuth with a reduced alpha dose for use in the production of semiconductors, and to a low alpha-ray emitting bismuth obtained with the foregoing method.
Generally speaking, bismuth has a low melting point of 271° C., and is used as a solder material as with lead and tin. Solder is used in: the production of semiconductors for bonding a semiconductor chip and a substrate; or bonding or sealing a Si chip for an IC, an LSI or the like with a lead frame or a ceramic package; or forming bumps during TAB (Tape Automated Bonding) or during production of flip chips, or is used, for example, as a wiring material for use in semiconductors. Moreover, in recent years, development as a thermoelectric material is also being conducted.
Recent semiconductor devices are of high density and with reduced operating voltage and cell volume, and therefore are subject to increased risk of soft errors caused by the effects of alpha rays emitted from materials in the vicinity of semiconductor chips. In particular, there is a strong demand for higher purification of solder materials, and there is a demand for low alpha-ray emitting materials.
While several main lead-free solder materials for use in semiconductors can be considered, tin-indium alloy and tin-bismuth alloy are being considered as a low alpha-ray emitting solder material for use in low temperatures. Nevertheless, since indium is extremely expensive, tin-bismuth alloy is regarded as the most likely prospect.
However, when a tin-bismuth alloy material is selected, it is necessary to reduce the alpha dose of both tin and bismuth. Conventionally, while technologies for reducing the alpha rays emitted from tin and lead have been disclosed, the current situation is that no research is being conducted for reducing the alpha rays emitted from bismuth.
While the present invention aims to provide low alpha-ray emitting bismuth, since the major usage thereof is as a solder material, technologies for reducing the alpha rays emitted from tin as a solder material are introduced below by way of reference.
Patent Document 1 below describes a method of producing low alpha-ray emitting tin by alloying tin and lead having an alpha dose of 10 cph/cm2 or less, and thereafter performing refining in order to eliminate the lead contained in the tin. The object of this technology is to reduce the alpha dose by adding high purity Pb and thereby diluting 210Pb contained in the tin.
Nevertheless, in the foregoing case, a complicated process of once adding Pb to tin and thereafter eliminating Pb is required. Moreover, while the alpha dose is considerably reduced 3 years after refining tin, this can also be understood to mean that the tin having a low alpha dose cannot be used in less than 3 years from the refinement, and it cannot be said that Patent Document 1 is an industrially efficient method.
Patent Document 2 describes that, by adding 10 to 5000 ppm of a material selected from Na, Sr, K, Cr, Nb, Mn, V, Ta, Si, Zr, and Ba to a Sn—Pb alloy solder material, the count of radiation alpha particles can be reduced to 0.5 cph/cm2 or less.
Nevertheless, even when these materials are added, the reduced count of the radiation alpha particles can only be a 0.015 cph/cm2 level, and this is lower than the level that is expected as a material for use in modern-day semiconductor devices.
An additional problem is that, as the additive materials, used are elements such as alkali metal elements, transition metal elements and heavy metal elements, of which inclusion into semiconductors is undesirable. Accordingly, there is no choice but to say that the material of Patent Document 2 is of a low level as a material for use in the assembly of semiconductor devices.
Patent Document 3 below describes reducing the count of radiation alpha particles emitted from solder extra fine wires to 0.5 cph/cm2 or less so that they can be used as the connecting wires of semiconductor devices. Nevertheless, this kind of count level of radiation alpha particles is lower than the level that is expected as a material for use in modern-day semiconductor devices.
Patent Document 4 below describes obtaining high purity tin having a low lead concentration and in which the alpha ray count of lead is 0.005 cph/cm2 or less by using special grade sulfuric acid or special grade hydrochloric acid (highly refined sulfuric acid or highly refined hydrochloric acid) as the electrolyte, and using high purity tin as the anode to perform electrolysis. If a high purity raw material (reagent) is used without regard to cost, it is obvious that a high purity material can be obtained. Even then, the lowest alpha ray count of the precipitated tin disclosed in the Examples of Patent Document 4 is 0.002 cph/cm2, and is unable to achieve the expected level despite the high cost.
Patent Document 5 below describes a method of obtaining metal tin having a purity of 5N or higher by adding nitric acid to a heated aqueous solution, in which coarse metal tin has been added, to precipitate metastannic acid; obtaining the metastannic acid through filtering and washing; dissolving the washed metastannic acid in hydrochloric acid or hydrofluoric acid; and using the obtained solution as the electrolyte to perform electrowinning. While this technology vaguely describes its application for use in semiconductor devices, no particular reference is made to the limitation of radioactive elements and the radiation alpha particle count, and it could be said that Patent Document 5 has a low level of interest regarding these issues.
Patent Document 6 below describes a technology of reducing the amount of Pb contained in Sn configuring the solder alloy, and using Bi or Sb, Ag, Zn as the alloy material. Nevertheless, in the foregoing case, even if Pb is reduced as much as possible, Patent Document 6 fails to particularly describe a fundamental solution to the problem of the radiation alpha particle count caused by the Pb that inevitably gets mixed in.
Patent Document 7 below discloses tin having a grade of 99.99% or higher and a radiation alpha particle count of 0.03 cph/cm2 or less which is produced by performing electrolysis using a special grade sulfuric acid reagent. In the foregoing case also, if a high purity raw material (reagent) is used without regard to cost, it is obvious that a high purity material can be obtained. Even then, the lowest alpha ray count of the precipitated tin disclosed in the Examples of Patent Document 7 is 0.003 cph/cm2, and is unable to achieve the expected level despite the high cost.
Patent Document 8 below describes lead, as a brazing filler material for use in semiconductor devices, having a grade of 4N or higher, a radioactive isotope of less than 50 ppm, and a radiation alpha particle count of 0.5 cph/cm2 or less. Moreover, Patent Document 9 below describes tin, as a brazing filler material for use in semiconductor devices, having a grade of 99.95% or higher, a radioactive isotope of less than 30 ppm, and a radiation alpha particle count of 0.2 cph/cm2 or less.
Both of the foregoing technologies have a moderate tolerance of the radiation alpha particle count, and there is a problem in that they are unable to achieve the level that is expected as a material for use in modern-day semiconductor devices.
In light of the foregoing circumstances, the present Applicant proposed high purity tin as described in Patent Document 10 below; that is, high purity tin, which has a purity of 5N or higher (excluding gas components of O, C, N, H, S, and P), and in which the contents of U and Th as radioactive elements are each 5 ppb or less, and the contents of Pb and Bi emitting radiation alpha particles are each 1 ppm or less, so as to eliminate, as much as possible, the influence of alpha rays on semiconductor chips.
In the foregoing case, the high purity tin is produced by being ultimately subject to melting and casting, and subject to rolling and cutting as needed, and Patent Document 10 relates to a technology for causing the alpha ray count of high purity tin to be 0.001 cph/cm2 or less.
Upon refining Sn, when it is heated during a production process such as melting or casting, Po becomes sublimed since Po has extremely high sublimability. If it is possible to eliminate the polonium isotope 210Po at the initial stage of production, disintegration from the polonium isotope 210Po to the lead isotope 206Pb can naturally be prevented, and it is considered that the generation of alpha rays can also be avoided.
This is because the generation of alpha rays during the production process is considered to be during the disintegration from the polonium isotope 210Po to the lead isotope 206Pb. Nevertheless, in reality, while it was considered that Po had mostly disappeared during production, the generation of alpha rays was still observed. Accordingly, it could not be said that simply reducing the alpha ray count of high purity tin at the initial stage of production would be the fundamental solution to the problem.
In light of the foregoing circumstances, the present inventors developed tin in which the alpha dose of the sample after melting and casting is less than 0.0005 cph/cm2 (see Patent Document 11). This tin can be obtained by leaching raw material tin having a purity level of 3N with hydrochloric acid or sulfuric acid, and thereafter performing electrolytic refining by using an electrolyte having a pH of 1.0 or less and Sn concentration of 200 g/L or less.
This technology is extremely effective, and it was possible to resolve the problems regarding tin, but bismuth still remained a material with a high alpha dose, and the problems regarding bismuth were still unresolved.
Meanwhile, there are the following Patent Documents as technologies related to bismuth.
Patent Document 12 relates to an electrolytic production method of tin sulfate and bismuth sulfate for tin-bismuth alloy plating, and discloses an electrolytic production method of tin sulfate and bismuth sulfate for tin-bismuth alloy plating, characterized in that: tin or bismuth as an anode is subject to melting in a sulfuric acid electrolyte by using an electrolytic bath in which the anode and the cathode are separated with an anion exchange membrane or with an anion exchange membrane and a cation exchange membrane, using a sulfuric acid solution as the electrolyte, and applying a direct voltage to the anode and the cathode; and a film plated with the obtained tin or bismuth salt has the radioactive alpha particle count of less than 0.1 cph/cm2.
Moreover, Patent Document 13 discloses a method of producing high purity bismuth via electrolytic refining by using an electrolyte containing hydrofluosilicic acid. Patent Document 14 describes a method of producing high purity bismuth via vacuum melting and vacuum distillation, and the device therefor. Patent Document 15 discloses a solder bonding method and an electronic device. Patent Document 16 discloses a method of producing Bi-212 via solvent extraction, and the device therefor, as well as the usage thereof.
Furthermore, Patent Document 17 relates to an electrolytic refining method for bismuth, and discloses an electrolytic refining method for bismuth, wherein bismuth metal in which the lead grade is controlled in advance to be 1 mass % or less is used as the anode, a titanium plate is used as the cathode, and hydrochloric acid solution is used as the electrolyte to perform electrolytic refining of bismuth in the hydrochloric acid solution at 10 to 30 g/L and at a current density of 150 A/m2 or less so as to enable electrolysis in a state where the tank voltage is stable, and thereby the refined bismuth in which the lead grade in the cathode electrodeposit is 0.01 mass % or less can be obtained.
Nevertheless, with this electrolytic refining method for bismuth which uses a hydrochloric acid bath, while it is effective in terms of eliminating lead, there is a problem in that the equipment becomes corroded since a bath with a high hydrochloric acid concentration is used.
With regard to Patent Documents 12 to 17 described above, while there are technologies for achieving the high purification of bismuth, the alpha ray count of the highly purified bismuth is of a 0.1 cph/cm2 level, and it was considered that this was the limit of the conventional technologies related to bismuth. Naturally, when a bismuth material obtained from these technologies is used, there is a problem of increased risk of soft errors caused by the effects of alpha rays emitted from materials in the vicinity of semiconductor chips.
Moreover, Patent Document 18 below discloses a technology of eliminating alpha-ray emitting nuclides by decreasing the nitric acid concentration of a commercially available product, in which crystals of bismuth nitrate are dissolved in a nitric acid aqueous solution, to coprecipitate bismuth oxynitrate and alpha-ray emitting nuclides. Nevertheless, since bismuth will also disappear and inferior efficiency will inevitably arise, there is a problem in that the production efficiency will deteriorate.
Moreover, as described in Patent Document 19 below, bismuth is normally refined via the distillation method or electrolysis, but with the distillation method, distillation needs to be repeated many times, and, when there are azeotropic mixtures, it is difficult to isolate and refine bismuth, and lead cannot be reduced to a level of 1 ppm or less. Moreover, the electrolyte, in which hexafluorosilicic acid and acid are mixed, and an additive such as glue is added thereto, is used for electrolysis. The contamination of lead results from the hexafluorosilicic acid and the additive such as glue, and there is a limit in that lead can only be reduced to a several 10 ppm level.
Meanwhile, in an electrolyte of acid (hydrochloric acid or sulfuric acid) which does not use hexafluorosilicic acid or any additive, by controlling the pH, bismuth concentration in the electrolyte, electrolyte temperature, and current density, it is now possible to cause the content of lead to be 1 ppm or less, contents of uranium and thorium to each be 5 ppb or less, and alpha dose to be 0.01 cph/cm2 or less (see Patent Document 19).
Moreover, the present inventors previously provided, as a method which is easier to handle than hydrochloric acid and sulfuric acid and which reduces the damage to the equipment, “a method of producing low alpha-ray emitting bismuth by inserting a titanium cathode and a bismuth anode in a nitric acid solution having a bismuth concentration of 5 to 50 g/L and a pH of 0.0 to 0.4 to perform electrolytic refining at a cathode current density of 0.1 to 1 A/dm2, and additionally subjecting the bismuth obtained from the electrolytic refining to hydrogen reduction melting or vacuum melting”; whereby it is possible to obtain bismuth having an alpha dose of 0.01 cph/cm2 or less from a raw material having an alpha dose of 0.05 cph/cm2.
Nevertheless, the alpha dose emitted from the bismuth obtained from Patent Document 19 and by the refining method using a nitric acid bath is 0.01 cph/cm2 or less; but in the case of using a material in which the alpha dose emitted from the bismuth raw material used in the electrolytic refining is high, it was discovered that an alpha dose that is higher than 0.01 cph/cm2 is generated after electrolytic refining, and it was necessary to make further improvements so that the alpha rays can be easily reduced even when using a raw material having a high alpha dose.
Moreover, it is now known that the alpha ray source in the bismuth raw material is mainly polonium. Polonium is a representative radioactive element that is contained in a bismuth raw material. In order to reduce the alpha dose, it is necessary to reduce polonium, but this point is not described in Patent Document 19.
The present invention provides a method of producing bismuth having a lower alpha dose even from a bismuth raw material having an alpha dose that is even higher than the bismuth raw material used in the foregoing electrolytic refining.